Property Dispute of House

My grandfather had brought a property on the name of his son who was an army officer as the mentioned property was in an army area. The money was paid by him and till my uncle was in service my grandfather along with my younger uncle lived in the same house till his death. My grandmother also resided there till death. My grandfather has also been taking rent on this property earlier in his account via cheque. He at his death left a document which stated his death rituals as well as his wish for the division of the house and construction of the second floor on the same. It also had the signature of two witnesses who are our neighbours. The will was implemented rightfully and our family, that included three brothers i.e. my father, his younger brother, and the yuoungest brother have been residing here since then as wished by my grandfather. Now after the death of my grandmother and the sudden increase in property we, that is my father and my youngest uncle, were sent a legal notice by my uncle, to vacate the house as the property is on his name. Now the case is in court. We have been residing here since 1996 after my father's retirement. What is your advice in this case?